The problem of providing a mechanical device which will provide accurate positioning of an object in a particular location, or will produce guided motion along one or more axes, is frequently encountered in mechanical engineering in general and in the design of scientific instruments in particular. In scanned image microscopy, a point source of radiaton and a single detector are used, and the object or some part of the microscope itself is moved so that the image is recreated point by point, either in a computer memory or on a cathode ray tube display, rather than producing a complete image at any given instant as in conventional optical microscopy. Such instruments clearly require a precisely constructed mechanical device which can move the object, or an appropriate part of the microscope, in a closely controlled way, usually in two dimensions, and generally at quite high speeds in order that the time taken to build a complete picture is not excessive. There are many known examples of such devices, incorporating a wide variety of techniques. The simplest way of guiding motion along one axis is to employ a sliding carriage mounted between two guides or slides, which themselvs can incorporate a wide variety of bearings. Motion along a second axis may be allowed by mounting the first set of guides on the carriage of a second set positioned at right angles to the first. The slides can be driven by rotating screws or linear positioners of any type. It is known to be possible to adapt such mechanisms to operate on very small scales, and to provide a highly controlled motion of small amplitude to drive the carriages by means of reducing mechanisms such as gears or levers, or electromagnetic or piezoelectric transducers. Such devices are frequently encountered in scientific instruments of various kinds. However, mechanical devices of this kind are prone to develop wear which can impair the accuracy of the positioning, and suffer from friction which can lead to sticking, making it difficult to produce a small displacement slowly and without overshoot. They usually require lubrication, and in some cases such as use in a high vacuum system or where it is important to ensure that there is no risk of chemical contamination, this can preclude their use. They can also be expensive to manufacture, because they involve a large number of moving parts which require precise machining. A need exists, therefore, for a simple mechanism which is capable of guiding motion along one or more axes which involves no parts in sliding contact with each other and is capable of fast vibration free positioning of an object and which is adapted for use in scientific instruments.
A simple device which is often used to guide a limited motion along a particular axis is the parallel motion solid hinge. In its simplest form, this consists of a fixed support to which two parallel thin arms of equal length are attached, and a moving block attached to the free ends of the arms. The rest position of the block is such that the arms are straight and at right angles to the support and the block. If a force is applied to the block at right angles to the arms and in the plane in which they both lie, the block will move in the direction of the force so that both arms are deflected, which also results in the block moving slightly closer to the support. If the arms are made of a resilient material, the block will return to its rest position with the arms straight as soon as the deflecting force is removed. If the arms are made from material with a rectangular cross-section, the block will experience a powerful restoring force if it moves out of the plane of the applied force because of the resistance of the arms to bending across the long side of the rectangular section, and thus no additional guides are necessary to keep the motion in the plane of the applied force. The slight inward motion away from the axis of the applied force as the block is deflected is put to a practical use in the construction of narrow resolving slits for spectrometers, which require very precise adjustment of the width, but the parallel motion solid hinge is also used to guide motion along the axis of the applied force, because this deviation from the axis of the force is very small if the extent of the motion is small in comparison with the length of the arms. It is especially suitable for providing an oscillatory motion, because of the presence of the restoring force due to the resilience of the arms, and the absence of any frictional forces. Despite these advantages, however, it is limited in application because of the very limited movement which is possible before the deviation from true axial movement becomes unacceptable. This can be reduced by employing very long hinge arms, but the longer these arms are the weaker is the resistance to movement out of the desired plane of motion, which may also be a serious limitation on the use of the device.
It is an object of the present invention, therefore, to provide a substantially frictionless mechanical positioning device based on the principle of the parallel motion solid hinge which is capable of guiding motion accurately along a particular axis over its entire working range, and one in which the working range is very much greater than a simple hinge of comparable dimensions. It is a further object to provide a substantially frictionless mechanical positioning device which is capable of the independent simultaneous positioning of an object along two axes. It is a further object of the invention to provide a scanning microscope using any form of incident radiation which incorporates a mechanical scanning device based on the positioning device described, and it is another object of the invention to provide a combined optical and scanning acoustic microscope incorporating such a scanner the construction of which would otherwise have been impossible.